The present invention relates to an apparatus that records information on an optical information recording medium and/or reproduces information from an optical information recording medium by means of holography.
Today, an optical disc with a recording density of about 50 GB is commercialized for consumer's use based on the Blu-ray Disc (BD) standard using blue-violet semiconductor laser.
In future, the capacity of an optical disc will be increased to the capacity equivalent to that of an HDD(Hard Disc Drive) that has a capacity of 100 GB to 1 TB.
However, the implementation of such a very high-density optical disc requires a new storage technology different from the trend in the conventional high-density technology based on short-wavelengths and high-NA objective lens.
Much effort has been made to study the next-generation storage technologies, and one of those technologies is the hologram recording technology that records digital information by means of holography.
A hologram recording technology is disclosed, for example, in JP-A-2004-272268. This document describes the so-called angular multiplex-recording method in which the signal luminous flux is condensed on an optical information recording medium through a lens and, at the same time, the reference beam of the parallel luminous flux is focused thereon to produce interference for hologram recording and, in addition, different page data is displayed on the spatial light modulator for multiplex recording while changing the incident angle of the reference beam onto the optical recording medium. In addition, this document describes the technology that can shorten the interval between neighboring holograms by condensing the signal beam through a lens and providing an aperture (spatial filter) in the beam waist and thereby increase the recording density/capacity as compared to that of the conventional angular multiplex-recording method.
Another hologram recording technology is described, for example, in WO2004/102542. This document describes an example of the shift multiplexing method in which, with the beams from inner pixels in one spatial light modulator as the signal beam and the beams from outer, bracelet-like pixels as the reference beam, both luminous fluxes are condensed on an optical recording medium through the same lens to produce interference between the signal beam and the reference beam near the focal point surface of the lens for hologram recording.
An encoding method for the hologram recording described above is disclosed, for example, in JP-A-9-197947. This document describes a two-dimensional encoding method for hologram recording that determines recording information by passing at least one light-wave through the two-dimensional spatial light modulator, wherein, with the neighboring four, or a multiple of four, pixels of the two-dimensional spatial light modulator combined into one set, a quarter of the pixels of each set pass the light and the three quarters of the pixels block the light.